Under certain operating conditions, internal combustion (IC) engines are subject to knocking or pinging. These are audio frequency-type oscillations of the compressed fuel-air mixture, triggered by a pulse wave. Heat transfer to the inner walls of the cylinders and of the piston is excessive during such oscillations, and may lead to damage due to thermal overload. Thus, knocking is to be avoided if at all possible. Yet, operation of an IC engine just below the knocking limit provides for most efficient use of the engine with respect to fuel consumption and other operating parameters. Thus, it is desirable to operate the engine just below the knocking limit. This, however, requires that any knocking or tendency to knock is to be recognized promptly and reliably, so that the combustion conditions can be changed to prevent such knocking by controlling the engine to operate below the knocking conditions. Knocking or pinging can be prevented by, for example, slightly retarding the timing of ignition, for example by retarding the spark.
It is difficult to obtain signals which clearly identify combustion conditions resulting in knocking or a tendency to knock distinguished from normal combustion operation since vibrations and various conditions within the engine result in generation of noise signals, from which selection of actual knocking signals is difficult. It is, therefore, a substantial problem to select, reliably, a signal indicating knocking without regard to disturbances and noise, so that a clear "knocking yes/no" signal can be obtained.
U.S. Pat. No. 3,540,262 describes a knock sensor which furnishes a knocking signal which is compared with a threshold level, independent of the engine signals. Upon exceeding the threshold, a knock recognition signal is provided. In practice, it has been found that such an arrangement has a disadvantage in that the reference is unrelated to background noise levels occurring in signals provided by the sensor, and due to operation of the engine itself. Comparison is carried out only with respect to an external signal, independent of motor or engine operation or condition.
U.S. Pat. No. 4,012,942 describes a knock sensor in which the knocking signal which is furnished by a knock sensor is compared with a reference, which reference is generated by a function generator operating in dependence on engine speed. Actually occurring background noise signals of the engine are not considered in this comparison; rather, it is the intention to simulate noise signals by providing the function signals of the function generator. This system is not responsive to particular instantaneously occurring operation conditions of the engine, its adjustments and characteristics, as well changes in design operation due to aging, wear and tear, and the like.
It has previously been proposed--see German patent Disclosure Document DE-OS No. 29 18 420--to compare a knocking signal furnished by a sensor and which, simultaneously, includes background noise signals in a comparator to which the signal is directly applied and additionally applied via a low-pass filter. In this system, the knocking signal together with background noise is compared with the signal which was derived from the last few cycles of the engine. In accordance with this disclosure, the actual measurement is carried out during a strobing interval, that is, by providing a certain measuring window related to a predetermined angular position of the crankshaft of the engine, so that the time during which the knocking signal can be sensed is positively correlated to a predetermined angular range of the crankshaft.